Inhaled manganese: Manganese toxicity may result in multiple neurologic problems and is a well-recognized health hazard for people who inhale manganese dust. Unlike ingested manganese, inhaled manganese is transported directly to the brain before it can be metabolized in the liver. The symptoms of manganese toxicity generally appear slowly over a period of months to years. In its worst form, manganese toxicity can result in a permanent neurological disorder with symptoms similar to those of Parkinson's disease, including tremors, difficulty walking, and facial muscle spasms. This syndrome is sometimes preceded by psychiatric symptoms, such as irritability, aggressiveness, and even hallucinations.

Methylcyclopentadienyl manganese tricarbonyl (MMT): MMT is a manganese-containing compound used in gasoline as an anti-knock additive. Although it has been used for this purpose in Canada for more than 20 years, uncertainty about adverse health effects from inhaled exhaust emissions kept the U.S. Environmental Protection Agency (EPA) from approving its use in unleaded gasoline. In 1995, a U.S. court decision made MMT available for widespread use in unleaded gasoline. A recent study in Montreal, where MMT had been used for more than 10 years, found airborne manganese levels to be similar to those in areas where MMT was not used. However, the impact of long-term exposure to low levels of MMT combustion products has not been thoroughly evaluated and will require additional study (26).

Ingested manganese: Limited evidence suggests that high manganese intakes from drinking water may be associated with neurological symptoms similar to those of Parkinson's disease. Severe neurological symptoms were reported in 25 people who drank water contaminated with manganese and probably other contaminants from dry cell batteries for 2-3 months. Water manganese levels were found to be 14 mg/liter almost 2 months after symptoms began and may have already been declining (1). A study of older adults in Greece found a high prevalence of neurological symptoms in those exposed to water manganese levels of 1.8-2.3 mg/liter, while a study of people in Germany drinking water with manganese levels ranging from 0.3-2.2 mg/liter found no evidence of increased neurological symptoms compared to those drinking water containing less than 0.05 mg/liter. Manganese in drinking water may be more bioavailable than manganese in food. However, none of the studies measured dietary manganese, so total manganese intake in these cases is unknown. In the U.S., the EPA recommends 0.05 mg/liter as the maximum allowable manganese concentration in drinking water.

A single case of manganese toxicity was reported in a person who took large amounts of mineral supplements for years, while another case was reported as a result of taking a Chinese herbal supplement. Manganese toxicity resulting from foods alone has not been reported in humans, even though certain vegetarian diets could provide up 20 mg/day of manganese.